This invention relates in general to precision reference sources and, in particular, to fast recovery temperature compensated precision voltage reference sources.
Many applications require precision voltage reference sources that exhibit fast recovery from transient events common to many system environments. It is important that these precision sources recover quickly from line perturbations, load switching and other events that can cause a transient deviation from normal operating conditions. It is often a requirement that signal processing circuits have a stable reference voltage circuit capable of providing an output voltage that remains relatively constant over a wide range of ambient operating conditions, including changes in temperature. Stable voltage sources are needed in power supply circuits and in personal computers where power to the precision supply or to the system blocks that load the supply is reduced or turned off. When it is desired to return to full power, the user wants to reach full system operation as soon as practically possible.
For this purpose, in the past, others have employed semiconductor voltage elements such as Zener diodes as a primary component of the voltage reference circuit. The Zener diode is typically operated in a reverse bias condition and voltage variations due to temperature are compensated with additional circuitry. One example of a precision voltage reference circuit is found in U.S. Pat. No. 5,300,877 assigned to the same assignee as this invention. In the referenced patent, a reverse biased Zener is coupled to a bridge resistor. The temperature variation in output voltage is compensated by two temperature compensation circuits, one circuit that provides a positive temperature coefficient compensation and another that provides negative temperature coefficient compensation. In both of the temperature compensation circuits, a reversed biased Zener diode is used as a reference source. The reverse biased Zener diode exhibits a positive temperature coefficient. That positive temperature coefficient is balanced by other devices exhibiting negative temperature coefficients.
As shown in particular in FIG. 4 and as discussed in column 6, line 7-16, two NPN transistors are coupled as transdiodes to provide temperature compensation for a series connected reverse biased Zener diode. The transdiodes have their bases shorted to their collectors and have a V.sub.be with a negative temperature coefficient. By selecting suitable operating points for these transdiodes and coupling them with a reverse biased Zener diode, the temperature coefficient of the circuit can be adjusted to have either a net positive or a net negative temperature coefficient. The reference also teaches how the output voltage of the circuit can be placed across a voltage divider with trimmable resistors and how the resistors can be trimmed at different temperatures in order to provide a relatively flat temperature coefficient over the operating range of the device.
However, devices such as those described in U.S. Pat. No. 5,300,877 have certain undesirable characteristics. The transdiode circuitry that makes up the negative temperature coefficient element may saturate when either the line voltage, V.sub.cc, or the output loading varies sufficiently to cause transient current spikes through the reference element circuitry. Some environmental stresses, such as gamma radiation, can also cause these current spikes resulting in transdiode saturation and delayed recovery to normal operation.
Accordingly, it is desirable to have a precision temperature compensated voltage reference circuit that depends on elements which exhibit a fast recovery in the face of transient upset events such as line perturbations, load switching and gamma radiation.